FIG. 1 shows a diagram of a typical system for recording ridge patterns. A source of light 1 radiates onto a component 2 which determines the position of the scanning surface 3 for the subject to be recorded, such as the ridge lines on the finger or the palm of the hand. On the scanning surface, the luminous flux from the source of light ends up carrying an image of this ridge pattern on the basis of the differences in the reflection of areas corresponding to the troughs and peaks of the ridge pattern. The optical system, as a rule including a collecting lens 4, a system of mirrors 5, an objective lens 6, protective glass 7 and microlenses 8 over the image sensor, takes this flux and creates an image of the ridge pattern on the light-sensitive surface 9 of a multi-element image sensor. The image sensor converts the image from an optical image into an electronic digital image in the form of an array of intensity values proportional to the radiant flux incident on the corresponding light-sensitive element, and transmits this image to the electronic memory 10. The processing unit 11 standardizes the scale of this electronic image, thus creating the output image of the system.
The component which determines the position of the subject to be recorded is, as a rule, designed as an optically transparent isosceles rectangular prism. However, there are variants in the design of the system for recording ridge patterns in which prisms of complex form, cylindrical components or plane-parallel plates act as the component determining the position of the scanning surface. In rarer variants, the body element of the system is the component determining the position of the scanning surface.
The number of mirrors in the optical system may vary and determines the shape and overall dimensions of the system.
The radiation sensor, as a rule, is constructed as a bar or matrix of metal oxide semiconductor transistors or charge-coupled devices.
One general disadvantage of said systems, as a consequence of very strict requirements on the quality of the image, is the necessity of using image sensors with relatively large light-sensitive elements, which leads to a considerable overall area of the sensor working surface and, as a consequence, to an extremely high cost of systems constructed with their use.
The considerable price of large-area sensors is due to the high cost of the silicon wafers from which they are manufactured and the low useful yield percentage of these wafers.
Thus, FIG. 2a shows the arrangement on the 150 mm diameter silicon wafer 12 of crystals 13 for a typical image sensor for a system for recording the ridge pattern of the palm of the hand with a distribution of 1000 dpi. A sensor of this kind has dimensions for its light-sensitive elements of 6.8 micrometers and contains 7216 elements along the horizontal and 5412 along the vertical. It may be seen from the figure that only four crystals of this nature may be accommodated on the wafer. Moreover; in this case, the useful area of the wafer usable for the manufacture of crystals accounts for around 50% of its total area. If a total of four critical production faults 14 are permitted during manufacture, but these are arranged as shown in FIG. 2a, for example, then not one serviceable crystal will be obtained from the wafer.
If a sensor is built with the same number of light-sensitive elements, but 1.4 micrometers in size, then the arrangement of the crystals on the 150 mm diameter wafer 15 may be, for example, as illustrated in FIG. 2b. In this case, the wafer accommodates 137 crystals 16 which occupy as much as 80% of the area of the wafer. At the same time, if a total of four critical production faults 17 are permitted during manufacture, arranged as shown in FIG. 2a, then 133 serviceable crystals will be obtained from the wafer. The losses due to defects thereby amount to just 3% of the total number of crystals on the wafer.
However, notwithstanding the obvious advantages, the use in systems for recording ridge patterns of sensors with small light-sensitive elements is constrained by the quality of the image formed, which is inadequate for compliance with current standards in the field of biometrics, particularly by noise and diffusion of the charge between the elements. FBI EBTS Appendix F is currently a key standard for ridge pattern recording systems.
There are a few variants for the design of systems for recording ridge patterns which bring about the required resolution and size of the scanning field whilst using relatively cheap image sensors.
Thus, U.S. Pat. No. 5,859,420, dated Dec. 1, 1999, classified under IPC G01B11/124, discloses a system in which the resolution of the system for recording ridge patterns is increased by subdividing the systems into a plurality of channels, each of which forms a separate part of the image of the subject to be recorded, after which the parts of the image are combined into the output image.
U.S. Pat. No. 6,928,195, dated Sep. 8, 2005, classified under IPC G06K9/32, discloses a system allowing an increase in the resolution of a system for recording ridge patterns without increasing the number of light-sensitive elements in the image sensor, by using a nutating mirror in the system to create a plurality of displaced intermediate images and forming an output image in which the elements of the intermediate images are interlaced.
This system is the closest analogue to the proposed invention. Its chief drawback is the presence of further elements and procedures which, although permitting the use of a relatively inexpensive sensor, do themselves make an additional contribution to the expense of the system and lower its reliability. As a consequence, a substantial reduction in the total cost of the system is not achieved, while at the same time reliability is reduced, the overall size is increased, the energy consumption is greater and the operating speed of the system is slower.